A transcriptional activation domain (TAD) is the region of a transcription factor (TF) protein that is necessary for its transcriptional activation activity when bound to a promoter. The TAD can be present at any location in the protein. These sequences are usually transportable, that is, they retain activation function when isolated from their native proteins and fused to any sequence specific DNA binding domain (DBD) protein. Hence, a TAD:DBD fusion can be used to turn on the expression of any desired target gene, when the promoter of that target gene contains a specific DNA sequence bound by the DBD. This property of TADs extends their utility in various agriculture and medicinal research. TADs are routinely being used in the study of protein-protein and protein-DNA interactions, and also being used for the targeted induction of genes in plants, animals and yeast.
TADs can be classified into three major classes depending upon their amino acid composition: proline-rich, glutamine-rich and acidic-rich. Most well characterized TADs, which confer strong transcriptional activation potential, including the yeast activator protein GAL4 and the VP16 protein from herpes simplex virus, fall in the category of acidic activators. These activation domains, though they are typically large in size, are routinely used for inducing gene expression, and for, protein-protein and protein-DNA interaction studies in yeast, plants and other animal science research.
The acidic activators form an amphipathic structure, that is, the activation domain contains many acidic and polar amino acids residues interspersed with hydrophobic residues. Such stretches of acidic amino acids are widely distributed in various proteins, but all regions rich in acidic amino acids do not necessarily have role in activation. Due to the loose consensus in the amino acid sequence conservation among activators, it is difficult to predict whether or not a given protein sequence has a role in transcriptional activation.
Activation domains that presently used in the art are generally derived from non-plant proteins such as GALA protein (yeast) and VP16 viral protein (herpes simplex virus). Due to their large size, fusion of these domains to a TF can lead to a change in the native structure which compromises the function of that TF. In addition, it may be considered undesirable to use sequences from non-plant proteins in plants destined for commercial use as transgenic crops, particularly those grown for food purposes.